1. Field of Invention
The present invention generally relates to fabrication of electronic circuit elements, and more particularly, to systems and methods for laser trimming of passive components fabricated on a substrate layer.
2. Description of Related Art
Resistors fabricated on substrate layers offer significant electrical performance advantages over surface mount resistors. The problem, however, is that printed circuit board and other circuit substrate manufacturers must fabricate these resistors on the substrate layer with the desired nominal and tolerance values at the time the substrate layer is being manufactured. Because printed resistors are typically buried within a printed circuit board or other circuit platform under one or more lamination layers, the platform manufacturer is generally unable to correct defects at a later time. The need to properly fabricate resistors the first time for both nominal and tolerance values has been an inhibiting factor for high yield and high volume buried resistor processes. These problems have become especially apparent for printed circuit boards having resistors printed on multiple layers. If resistors at one layer fail to meet the required specifications, the entire circuit board may be defective. As a result, multi-layered printed circuit boards and other circuit platforms such as multi-chip modules, hybrid circuits and chip packages typically suffer from exceptionally low yields.
Conventional approaches have attempted to alleviate these problems by performing laser trimming of resistors to conform the resistors to the required nominal and tolerance values. Generally, there are two basic ways to produce a change (higher ohms) in thick polymer resistors: (a) reducing or changing the path of current through the resistor in terms of magnitude and direction; and (b) reducing or changing the cross-sectional area perpendicular to the direction of current flow. The first method is typically performed by making a trim slice through a portion of the resistor to create a localized reduction in the cross-sectional area relative to the direction of current flow. This method, however, distorts the electrical field around the slice cut and can produce undesirable variations in the impedance of the resistor at higher frequency. The second approach performs a planar cut to reduce the cross-sectional area of the resistor in the direction of current flow. Because only the magnitude of the current is affected and not the direction, high frequency impedance will not be significantly affected.
Although some forms of trimming may perform well for the more traditional square resistors, the recent introduction of annular resistors presents unique problems. Due to the different physical geometries of annual resistors, conventional laser trimming equipment and processes may be unsuitable for laser trimming annular resistors. In addition, printed circuit board manufacturers may be unwilling to incur the cost of purchasing special-purpose laser trimming equipment, which can exceed one million dollars per laser trimming tool, or modify existing laser trimming equipment to perform laser trimming of annular resistors. For circuit substrate layers having a large number of buried resistors or resistors printed on multiple layers, conventional laser trimming processes can also be difficult and time consuming due to the need to select laser trimming settings for each resistor. Furthermore, because conventional laser trimming approaches typically do not collect and display statistical information in a meaningful way, the circuit board manufacturer may have insufficient information to adjust laser trimming process or the underlying print screening processes.
Therefore, in light of the problems associated with existing approaches, there is a need for improved systems and methods for laser trimming of components.